demos/cube: Explicitly specify std140 layout on UBOs
demos/cube: Explicitly specify std140 layout on UBOs

Without this, glslang won't actually provide the offsets and strides
required by SPIR-V.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools